This invention relates to the polymerization of 2-pyrrolidone and more particularly the the formation of particles* of this polymer which are suitable for melt extrustion processes by a method which is adaptable to continuous production. FNT *As used herein the words "particles" and "particulate" refer to material large enough to be retained on a 30 mesh sieve (U.S.A. Standard Sieve, A.S.T.M. designation E11, having an opening size of 600 micrometers) but which will ordinarily pass through a 6 mesh sieve having an opening size of 3.36 mm. Thw words "granules" and "granular" refer to material which will pass through a 30 mesh sieve.
The polymerization of 2-pyrrolidone using an alkali metal catalyst and an activator under anhydrous conditions is well-known in the art. Relative prior art is recited in the co-pending application Ser. No. 039,773. Most of the prior art discloses the method of polymerization known as "bulk polymerization" which results in the formation of an exceptionally hard, tough cake that is very difficult to grind. It must be ground, however, not only to facilitate the removal of the alkali and unpolymerized 2-pyrrolidone by washing, but also to obtain the polymer in a form which is useful for manufacturing processes, e.g. for feeding to a melt extruder. It is most desirable therefore to provide a method of making the polymer in particulate form by a direct process.
Several attempts have been made to accomplish this by means of suspension polymerization wherein the 2-pyrrolidone containing the alkaline catalyst and activator are added to a non-solvent under anhydrous conditions. The non-solvent most commonly employed is hexane or a similar hydrocarbon. The mixture is heated to the optimum polymerization temperature, usually about 50.degree. C., and then stirred to form droplets of the 2-pyrrolidone polymerization mixture which then polymerize as separate particles. The first disclosure of the suspension polymerization of 2-pyrrolidone was in U.S. Pat. No. 2,739,959, Example 13. The resulting product was not useful because it was obtained as a very fine powder. Also the state of the art was such at that time that very low molecular weight polymers were formed. As polymerization techniques improved and higher molecular weight polymers were produced it was found that the polymerizing mixture passes through a sticky plastic phase during which it adheres strongly to the stirrer and to the walls of the polymerization vessel. U.S. Pat. No. 2,999,852 discloses a method of avoiding this problem by coating the surfaces of the polymerization vessel and stirrer with alkyl trichlorosilanes. U.S. Pat. No. 3,213,066 discloses a method of polymerizing in a ball mill in an attempt to avoid these problems.
All of these processes result in a finely divided powder which is not useful for most manufacturing processes such as feeding to a melt extruder. A method which does produce particles of a larger size is disclosed in U.S. Pat. No. 4,017,465. The method involves the addition of an inorganic salt to the polymerization mixture together with a hydrocarbon such as hexane and stirring this mixture. In order to give satisfactory results, however, about four times as much salt as 2-pyrrolidone must be used. This volume of added salt, aside from its cost, reduces the productive capacity of the reactor thereby adding significantly to production costs. It would be desirable for economic reasons to eliminate both the added salt and the added non-solvent.
It is an object of this invention to provide an inexpensive method of forming particulate polymers of 2-pyrrolidone suitable for melt extrusion purposes.
It is further object of this invention to provide a method of forming particulate polypyrrolidone which lends itself to continuous commercial production.